A load-shiftable transmission, particularly in an automotive vehicle, has an input shaft connected with a drive source such as the internal-combustion engine and an output shaft connected to a load such as the wheels of the vehicle. Between the input shaft and the output shaft, there are provided a multiplicity of continuously meshing gears which cooperate with respective friction devices, such as friction clutches or brakes, which can be hydraulically actuated to limit relative rotation of parts of the drive or to couple parts of the drive together and thereby establish respective force-transmitting paths between the input and output shafts with different transmission ratios corresponding to the "speeds," "gears" or steps of the transmission. For convenience, these various transmission ratios will be referred to hereafter as "speeds."
It is known, for example, from U.S. Pat. No. 4,026,169 to provide an electric circuit for the electro-hydraulic actuation of such a transmission. To this end, each of the hydraulic devices, e.g. the friction clutches, is associated with an electromagnetic valve which, when energized, permits hydraulic filling of the device and thus selection of the associated speed of the transmission. The electromagnetic valves are, in turn, energized by an array of switches.
One of the problems with such a system is that a certain overlap of the switches of the friction devices is necessary to effect smooth transfer of the driving action from one bear train to another, e.g. the shifting of one gear train should overlap the release of the other gear train at least under some conditions.
To accomplish this with a system in which the manual or automatic shifting is effected by electrohydraulic means, it is necessary to provide for an overlap of the switched-on and switched-off states of the electromagnetic valves for the newly selected speed and the previous speed, respectively. This is accomplished by providing time delay switches which ensure a lag in opening the electromagnetic valve for the previous speed when the next speed is selected. Naturally, the overlappingly operated friction devices slip during this overlap. As a consequence a jolt-free transfer is effected in shifting from one speed to another.
The time-delay switch of this earlier system has, at the input side, working contacts connected to a common voltage source while the output side is connected to the respective electromagnetic valve. The working contacts are thus in a so-called parallel connection.
When such a circuit is damaged or disrupted such that two contacts remain closed, e.g. because one does not open sufficiently quickly because of a malfunction of the time-delay network, the associated clutches remain engaged for a corresponding duration so that two speeds may be practically rigidly connected between the input shaft and the load. Naturally this can damage the gear trains or destroy the clutches.